Add tooltipZIndex property

[ARandomDude]

I'm submitting a ...

[X] bug report
[ ] feature request 
[ ] support request

Plunkr Case (Bug Reports)
http://plnkr.co/edit/78IBU8g0UvINCmQ7aV26?p=preview

Current behavior
In a p-dialog with an input with a pTooltip, the tooltip gets shown on hover as expected; but when clicking the input, the tooltip moves behind the dialog.

Expected behavior
I would expect that, on clicking the input, the tooltip stays on top of the dialog.

Minimal reproduction of the problem with instructions
See the Plunkr above. Click the "dialog" button and then hover and then click the input.

As I can see, the problem is that everytime the dialog or a component inside it gets clicked, the p-dialog's z-index is increased by one (this is probably intended behavior; but leads to the above issue).

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Temporary workaround for other users: Just assign the tooltips a z-index high enough that the abovementioned increase will not overshadow the tooltip, e.g.:
body div.ui-widget.ui-tooltip { z-index: 3000 !important; }

[cagataycivici]

Added tooltipZIndex to tooltip directive to managing these easier as I currently think it is easier to handle on your side, defaults to auto and in your case should be a higher value.